1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for supplying reburning secondary air to the exhaust system of an internal combustion engine thereby to oxidize and clear the unburned noxious content such as hydrocarbons (HC) or carbon monoxide (CO) of the engine exhaust gas, and, more particularly, to a secondary air supply control device for use with the engine exhaust gas purifying apparatus, in which the flow rate of the secondary air to be supplied is properly controlled in accordance with the running state of the internal combustion engine thereby to ensure the effective purification of the exhaust gas and to prevent an abnormal combustion such as the afterburning phenomena from occurring and the exhaust pipe and/or the exhaust muffler of the exhaust system from allowing its surface to be discolored or to lose its gloss due to the excessive heating.
2. Description of the Prior Art
In the deceleration of an internal combustion engine, generally speaking, the unburned content in the exhaust gas has a tendency to be increased due to the reduction in the flow rate of intake air. If, in this state, excessive secondary air for the exhaust gas purification is supplied, that unburned content may be instantly burned to invite disadvantages that the so-called "afterburning phenomena" may be invited, that the exhaust pipe and/or the exhaust muffler may be heated to accelerate the discoloration or the loss of the gloss of its surface, and that the deterioration of a catalytic converter may be promoted.
When in the choking operation at the start of the internal combustion engine, moreover, the air-fuel ratio of the combustible mixture sucked has a tendency to be shifted to a rich side, and this tendency is promoted, in case the idling operation is continued for a long time while being left in the choking operation, thereby to allow the unburned content to be more discharged. This case is also accompanied by the same disadvantage as that during the aforementioned deceleration.